The present disclosure is directed to a method and apparatus for more efficient operation of a refrigeration system. More particularly, the disclosure relates to a method and apparatus for more efficient operation of a variable refrigerant chiller in the refrigeration system, which includes an additional refrigerant vessel to allow for variation of the amount of refrigerant in the refrigeration system.
Conventional chilled liquid systems in vapor compression refrigeration systems used in heating, ventilation and air conditioning systems include a condenser vessel, an evaporator vessel, a compressor, a variable speed drive (VSD), an expansion valve, and optionally a hot gas bypass valve. Operation of a chiller system produces chilled liquid (e.g. water) (Tch) at varying load and cooling tower conditions. To efficiently produce Tch, various compressor elements of the chiller system are employed.
In conventional refrigeration systems, the evaporator effects a transfer of thermal energy between the refrigerant of the system and another liquid to be cooled. As a result of the thermal energy transfer with the liquid, the heat is transferred into the refrigerant converting some of it into vapor, which is then returned to a compressor where the vapor is compressed, to begin another refrigerant cycle. The cooled liquid can be circulated to a plurality of heat exchangers located throughout a building. Warmer air from the building is passed over the heat exchangers where the cooled liquid is warmed, while cooling the air for the building. The liquid warmed by the building air is returned to the evaporator to repeat the process. During operation of the chiller, the liquid level is maintained in the chiller through a control loop utilizing the expansion (throttling) valve to control the height of the liquid level in the condenser vessel. The evaporator also has a mixture of liquid and gas refrigerant. The heat transfer characteristics in the evaporator are affected by the number of tubes “submerged” in the liquid refrigerant versus gas refrigerant.
Chiller operation is desired to control and produce Tch at a setpoint (e.g., 44 degrees F.) under different load conditions in the presence of disturbances such as low load scenarios, medium load scenarios, and high load scenarios. When considering a chiller for purchase there are load considerations that are used to estimate the peak load required to support the operation. This impacts the physical size of the chiller vessels, the number of tubes, size of compressor, and associated piping sizes. In addition, the refrigerant (e.g., R134a) charge is calculated based on the desired heat flux (BTU/hr*ft2) in the refrigerant system.
Conventional chilled liquid systems provide a fixed amount of refrigerant in the system and thus are only optimized for one operating condition or state. Although conventional chiller systems are designed to run efficiently, over time, the chiller systems are often not running as efficiently as they could be due to fouling or other factors. Thus, there exists a need for chiller systems with variable refrigerant control.
Another situation that is to be avoided in conventional chilled liquid systems is surge. Surge or surging is an unstable condition that may occur during centrifugal compressor operation. Surge is a transient phenomenon having oscillations in pressures and flow, and results in complete flow reversal through the compressor. Surging, if uncontrolled, can cause excessive vibrations in both the rotating and stationary components of the compressor, and may result in permanent compressor damage. One common technique to correct a surge condition may involve the opening of a hot gas bypass valve to return some of the discharge gas of the compressor to the compressor inlet to increase the flow at the compressor inlet.
Therefore, what is needed is a high-efficiency chiller system that allows for efficient chiller operation and that prevents surge during low load conditions. The addition of a variable amount of refrigerant in the system enables another degree of freedom for operation of the chiller by changing the heat transfer characteristics and refrigerant level in the evaporator vessel.